How to make sharp and rounded corners
If you're modeling using subdivision, at its simplest, this sort of topology will give you a rounded corner: supporting loops close to the sharp edges, and looser on the round corner. You have a degree of control over the tightness of the corner by sliding the edges on either side of it.

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However, if, for example, you you wanted a perfect circular arc in the corner, breaking instantly to perfectly orthogonal edges, you might choose a bevel, as suggested by atek.